In a typical wireless communications network, communication devices, also known as wireless devices, mobile stations, terminals, and/or User Equipments, UEs, communicate via an access network, e.g. a Radio Access Network, RAN, with one or more core networks. The access network may cover a geographical area which is divided into cell areas, with each cell area being served by a network node, e.g. an access point, AP, or a base station. A cell is a geographical area where radio coverage is provided by the network node at a node site, or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. One network node may have one or more cells. The network nodes communicate over the air interface operating on radio frequencies with the communication devices within range of the network nodes. Note that, hereinafter, network nodes or base stations may also be referred to as communication devices.
An Ultra Dense Network, UDN, comprise a large number of densely deployed radio base stations or network nodes. Hence, UDN network nodes have to possess a strong capability to handle the interference. Further, a UDN network conventionally operates over millimetre wave, mmW, frequencies because of the potential of using wide bandwidth. However, beamforming with a large number of antenna elements is normally required in this case in order to ensure the coverage as mmW frequencies experiences severe fading.
Based on these prerequisites, in order to get the channel information to enable beamforming, beacon signals needs to be transmitted according to predefined beam patterns. For example, a beacon signal sweeping with repeated information in different beam directions may be applied to cover the entire desired coverage range. Thereafter, data transmission may be transmitted using trained beams, which has a narrower beam-width than that of current wireless communications networks.
It should be noted that it is advantageous for the densely deployed UDN network nodes to be capable of adaptively cooperating in order to avoid interference for data transmissions in the UDN network. Interference avoidance in both intra-UDN networks and/or inter-UDN networks is also, in the context of dynamic beamforming based transmissions, important at a Media Access Control, MAC, level. Therefore, an efficient MAC protocol emphasizing this aspect is needed to be developed for UDN networks in order to meet the abovementioned requirements.
Currently, the IEEE-802.11 system dominates the unlicensed frequency band. It is valuable to have a brief view of IEEE-802.11 radio resource management when discussing MAC protocols for UDN networks.
In recent standards of IEEE 802.11, the communication devices are competing about the radio resources via a mechanism referred to as “listen before talk”. This means that a communication device may transmit a signal only when the detected transmission power level is lower than a certain predefined threshold. This is performed in order to avoid collision of signals. This may, for example, be implemented using the so-called Distributed Coordination Function, DCF, wherein each communication device waits a random back-off time before accessing the radio channel, thus allowing other communication devices to get a fair chance to access the radio channel. If a second communication device node has a random back-off time that becomes zero before the first communication device, the first communication device may determine that the second communication device has started transmitting. Consequently, the first communication device postpones its data transmission. At the next transmission possibility, the first communication device continues to count down the random back-off time until it is zero. When the back-off time expires, i.e. the back-off timer becomes zero, the first communication device attempts to perform its data transmission. Upon a collision, the communication devices may increase, e.g. up to a certain limit, their sensing time, e.g. back-off time, in order to avoid further collision to a large extent.
However, this also means that first and second communication device wanting to transmit data, thus contending for the radio channel, e.g. using the DCF or any other form of carrier sensing, and therefore are listening to the radio channel, may not be able to hear each other due to, for example, directive data transmissions of the other communication device. This may result in that, for example, if both the first and second communication devices want to communicate with the same third communication device, the first and second communication devices direct their respective radio transmission beams towards the common third communication device, whereby a collision occurs. This is an example of what is commonly referred to as the hidden node problem, i.e. that two transmitting communication devices are hidden from each other.
In order to reduce the hidden node problem, the above mechanism may be complemented by Request to Send, RTS, and Clear to Send, CTS, signalling. This signalling means that a communication device that intents to transmit data sends out an RTS signal, and only if it also receives a CTS it will start with the actual data transmission.
As indicated above, an IEEE-802.11 system using DCF (with or without RTS/CTS signalling) provides each communication device with an opportunity to get the radio resource. However, while such system works well when the data traffic is low, the radio resource efficiency will become low when the data traffic is high because of an increased number of collisions, i.e. higher data traffic infers an increased probability of collisions. In brief, it may be shown that the IEEE-802.11 using DCF does not scale too well with increasing number of contending communication devices and offered data traffic. This will most likely occur in UDN networks.
PCT/SE2013/051562 describes a MAC protocol for UDN networks. In this case, the radio resources are split between control and data for a contention-based MAC of the UDN network. That is, the total radio resource is split for control and data channels, where communication devices contend to reserve the resource blocks for data transmissions, while data-transmission is done at another separate radio resource portion other than that of the control channel. Hence, in most cases, when the data bandwidth is much greater than that of the control bandwidth, a minimized or reduce number of conflicts will occur on the data channel and most of the contention will instead happen at the narrow separated control channel. This results in a better overall channel efficiency.
However, while this MAC protocol provides a solution which at least partially mitigates the hidden node problem, it is still necessary to further provide a MAC protocol which may improve the interference management in a wireless communications network.